1. Field of the Invention
This invention relates to a spark plug, particularly to a spark plug that is installed to face into the combustion chamber of an internal combustion engine to ignite and burn an air-fuel mixture supplied into the combustion chamber and that is connected to an ion current detector for detecting ionic current arising during combustion of the air-fuel mixture.
2. Description of the Related Art
In a spark-ignition internal combustion engine, a high voltage generated by an ignition coil is applied through a distributor or the like to spark plugs installed in the individual cylinders. The spark discharge that the high voltages produces across the gap between the spark plug electrodes ignites the air-fuel mixture, causing combustion. However, when certain causes are present during the engine ignition/combustion stroke, the combustion of the air-fuel mixture does not proceed normally, i.e., misfire occurs.
When the air-fuel mixture burns normally, the combustion is accompanied by ionization of the air-fuel mixture (more precisely the combustion gas produced by normal burning of the air-fuel mixture). This generates ionic current at the gap between the center electrode and ground electrode of the spark plug. When misfire occurs and the air-fuel mixture does not burn, the air-fuel mixture does not ionize and no ionic current arises.
This has led to the common practice of detecting engine misfire by connecting an ion current detector to the spark plugs, detecting the ionic current produced in the combustion chambers at each combustion stroke, and comparing the detected value of the ionic current with a prescribed value.
Owing to the fact that the ionic current detection is conducted by detecting the value of the current generated at the gap between the center electrode and ground electrode of the spark plug in this manner, it is preferable for improving the detection accuracy to facilitate the flow of ionic current in the vicinity of the spark plug, particularly in the vicinity of the electrodes functioning as detection probes. The spark plug taught by Japanese Laid-Open Patent Application No. Hei 5(1993)-99956, for example, was developed for this purpose. In this spark plug, the surface area of a nickel (Ni) alloy center electrode exposed within the combustion chamber is defined to have an area of 25 mm2 or greater so as to expand the contact area with the ionized combustion gas and thus facilitate the flow of ionic current.
In contrast to this, however, use of a small-diameter center electrode is preferable from the aspect of spark plug ignition performance, particularly in the points of mitigating flame quenching effect, enhancing antifouling performance, improving the ignition limit during lean-burn operation and lowering the discharge voltage (i.e., lowering the voltage required for ignition on the engine side). FIG. 8 shows how required center electrode diameter varies with lean-burn limit air/fuel ratio. FIG. 9 shows how required center electrode diameter varies with voltage required at ignition.
Recent years have therefore seen a move toward replacing the nickel and platinum (Pt) conventionally used as the material of the center electrode with iridium (Ir), a metal characterized by high melting point, high hardness and outstanding corrosion resistance. Today, therefore, wide use is made of spark plugs that achieve long service life despite having very fine electrode diameters on the order of 0.4 mm to 0.7 mm. When nickel is used as the center electrode material, the diameter is generally on the order of 2.5 mm.
Since the aforesaid prior art facilitates the flow of ionic current by setting the area of the center electrode to a large value, it cannot easily improve ionic current detection accuracy while also reducing center electrode diameter but ensuring satisfactory ignition performance.